With the recent diffusion of computers, inkjet printers have been widely employed in printing on papers, films, fabrics and so on not only in offices but also in homes.
Inkjet recording methods include a system of jetting ink droplets under pressurization with the use of a piezo device, a system of foaming an ink by heating and thus jetting ink droplets, a system using ultrasonic wave, and a system of electrostatically sucking and jetting ink droplets. As ink compositions for inkjet recording by these systems, use is made of water-base inks, oil-base inks or solid (molten) inks. Among these inks, water-base inks are mainly employed from the viewpoints of production, handling properties, odor, safety and so on.
Requirements for a coloring agent to be used in these inkjet recording inks are as follows: being highly soluble in solvents, enabling high density recording, having a favorable color hue, having high fastness to light, heat, air, water and chemicals, having favorable fixation properties on an image receiving material with little bleeding, being excellent in storage properties as an ink, having no toxicity, having a high purity, and being available at a low cost. However, it is highly difficult to search for a coloring agent satisfying these requirements at a high level. Although various dyes and pigments have been already proposed and practically employed in inkjet recording, no coloring agent satisfying all of the above requirements has been found out yet. Using well known dyes and pigments typified by those having color indexes (C.I.) assigned thereto, it is highly difficult to satisfy both of such color hue and fastness as required in inkjet recording inks. Investigations have been made on dyes having favorable color hue and high fastness to develop excellent coloring agents for inkjet printing. However, it is unavoidable that a compound usable as a water-soluble dye has water-soluble substituents. There has been found out a problem that in the case of increasing the number of these water-soluble substituents in order to improve the stability of an ink, the resultant image is liable to bleed under highly humid conditions.
The present inventor has found out that use of a betaine compound is efficacious in overcoming this bleeding phenomenon.
In the case of using such a betaine compound alone, however, it is revealed that the bleeding can be relived but deposition of a color material arises in a high-density part and the thus formed image frequently suffers from bronzing.